Use flake8, gjslint and jshint

With this PR `gjsjint` and `jshint` are used to check the JavaScript code. And `flake8` is used to check the Python code.

Fixes #28.

This PR also fixes an error that occurs when compiling the project `build.js` file:

```
info ProjectSample Parsing dependencies
info ProjectSample Compiling 338 sources
ERR! compile ERROR - unknown @define variable goog.json.USE_NATIVE_JSON
ERR! compile
ERR! compile
ERR! compile 1 error(s), 0 warning(s),
ERR! compile 97.6
ERR! compile % typed
```